In recent years, multimedia services of various information provided by use of radio are becoming active, and a number of wireless apparatuses are developed and provided for practical use. The variety of services is being increased year after year to telephone, television, LAN (Local Area Network), and the like. To enjoy all of the services, the user has to have wireless apparatuses corresponding to the respective services.
To improve the convenience for the user to enjoy such services, movement of providing the multimedia services any time, any where without making the user aware of the existence of the media, that is, in a ubiquitous manner has started, and a so-called multi-mode apparatus realizing a plurality of information transmission services by itself is, though partially, realized.
Since normal ubiquitous information transmission services by radio use electromagnetic waves as a medium, in the same service area, one frequency is assigned per service, thereby providing a plurality of services to the user. Therefore, the multimedia apparatus has the function of transmitting/receiving electromagnetic waves of a plurality of frequencies.
In a conventional multimedia apparatus, for example, a method of preparing a plurality of single-mode antennas each corresponding to one frequency and mounting the antennas on a single wireless apparatus is employed. In the method, to make the single-mode antennas operate independently of each other, the single-mode antennas have to be mounted at intervals of about wavelength. The frequencies of electromagnetic waves used for services related to normal ubiquitous information transmission are limited to hundreds MHz to a few GHz by the free space propagation characteristic. Therefore, the distance between neighboring antennas becomes tens cm to a few meters, the dimensions of the apparatus become large, and portability for the user is not satisfied. Since the antennas having sensitivities to different frequencies are disposed at the intervals, RF circuits coupled to the antennas have to be also separated from each other and installed in correspondence with the different frequencies.
Therefore, it is difficult to apply a semiconductor integrated circuit technique. If the technique is applied, problems occur such that the dimensions of the apparatus become large and, in addition, the cost of the RF circuit increases. If the integrated circuit technique is forcefully applied to integrate all of the circuits, it is necessary to couple the RF circuit to an antenna apart from the RF circuit via an RF cable. The RF cable which can be applied to a terminal of dimensions small enough to be carried by the user has a diameter of about 1 mm. Consequently, the transmission loss of the RF cable reaches a few dB/m under present circumstances. The method has problems such that the consumption power of the RF circuit increases due to use of the RF cable, it causes noticeable reduction in use time of an apparatus providing ubiquitous information service or noticeable increase in the weight of the apparatus due to increase of the volume of a battery, and convenience for the user of the apparatus largely deteriorates.
As another technique, a two-frequency antenna such that one end of a loop antenna or the material of an antenna is coupled to a transmitter using a frequency and the other end is coupled to a receiver using another frequency is disclosed in Japanese Patent Laid-Open Nos. S61(1986)-265905 (Document 1) and H1(1989)-158805 (Document 2).
In the two-frequency antenna disclosed in the document 1, a first resonant circuit is connected to one of ends of a loop antenna as a radiating conductor and a second resonant circuit is connected to the other terminal. The one terminal resonates at a transmission frequency and the other terminal resonates at a reception frequency. A transmission circuit is connected to the one terminal (transmission output terminal) and a reception circuit is connected to the other terminal (reception input terminal).
In the two-frequency antenna disclosed in the document 2, a first resonant circuit which resonates at a transmission frequency and is connected between one of terminals of the material of an antenna as a radiating conductor and a transmission output terminal presents a high impedance at a reception frequency and disconnects the material of the antenna from the transmission output terminal. A second resonant circuit which resonates at a reception frequency and is connected between the other terminal of the material of the antenna and the reception input terminal presents a high impedance at the transmission frequency and disconnects the material of the antenna from the reception input terminal.